Over a period of nearly five decades, flow cytometry has evolved from a simple technique for counting suspended particles (e.g., analytes, cells or DNA) in fluid into a highly sophisticated and versatile technique that is critical to clinical diagnosis and fundamental biomedical research. Early efforts in the development of flow cytometry focused upon the attainment of a stable flow system able to transport particles, without disturbance by any alien aerosol, to regions of laser beam illumination for optical interrogation via fluorescence or light scattering. A standard approach for today's flow cytometers is to create a laminar sheath flow in a transparent capillary tube to minimize noise due to fluctuations in position and propagation speed of the particles. Besides such improvements in controlling particle flow and in flow cytometry instrumentation generally, significant progress has also been made with respect to other aspects of flow cytometry, for example, with respect to the methods of cell preparation, new fluorescent dyes and new markers of cell properties.
These technological advances in flow cytometry have made it possible to use flow cytometers in a variety of areas. For example, flow cytometers are now used for analyzing white blood cells in AIDS patients. Also for example, flow cytometers are now employed in performing cancer diagnosis and stem cell sorting. Indeed, flow cytometry is now widely recognized as an important clinical and research tool. However, even though the size of a flow cytometer has been reduced from a piece of equipment occupying an entire room to a table top system with ever increasing functionality and performance, flow cytometers continue to cost between $150K and $1 M, and consequently remain a tool affordable only by major medical centers and laboratories. Size and price reduction by orders of magnitude (e.g. 1000 times) are necessary to make flow cytometers a prevailing diagnosis tool that can be afforded by more hospitals and medical practitioners around the world.
One technique that holds promise for miniaturizing flow cytometers is the use of microfabricated flow cells, enabled by advances in microfluidics. Integrated microfluidic chips that perform a variety of functions for chemical analysis and biological screening have found wide applications in the pharmaceutical industry and have accelerated the progress of research in biotechnology. Several research groups have demonstrated the ability to manipulate cells and micro-particles in microfluidic devices using the effects of fluidic pressure dielectrophoresis, optical trapping, and electro-osmosis. More particularly, the introduction of microfabricated electrodes in the fluidic channels of microfluidic devices can facilitate the optical detection of particles by controlling and manipulating the positions, angles, and populations of the particles in microfluidic channels via the dielectrophoretic effect.
There are several reasons that make these results particularly relevant to the development of compact flow cytometers. First, biological cell sizes fit well with the dimensions of the microfluidic devices that can be easily and precisely fabricated using microfabrication techniques such as lithography and molding. Second, microfluidic devices tend to support laminar flow, making the flow control simpler and fluid transport highly efficient. Third, micro-scale integration allows more functionality (e.g. pumps, valves and switches) to be incorporated into the device. Finally, two-dimensional or even three-dimensional array structures can be fabricated to enhance the performance of the system and alleviate the limit of device throughput.
Although rapid progress has been made in microfluidics that is applicable to flow cytometry, the scheme of optical detection employed in flow cytometry has not experienced similarly important advances or changes. In particular, while the hardware utilized in performing optical detection has continued to evolve, resulting in more advanced lasers, more sensitive detectors, and superior optical mechanical components, there nevertheless has not been any paradigm shift in terms of the manner in which optical detection is performed in flow cytometry. As a result, the expensive and bulky optical setup currently necessary for fluorescence detection threatens to become a bottleneck restricting the realization of compact, low-cost flow cytometers. Additionally, the relatively high cost of lasers and light detectors for use in flow cytometry is further exacerbated when one reduces the size of the overall system.
For at least these reasons, it would be advantageous if an improved optical detector, optical detection scheme or optical detection method could be developed for use in detecting small objects or particles such as cells or DNA, as could be used for, among other things, performing flow cytometry and related techniques. More particularly, it would be advantageous if, in at least some embodiments, such an improved optical detector/detection scheme/method could be designed in which smaller (in terms of size and/or weight) optical components could be employed. Additionally, it would be advantageous if, in at least some embodiments, simpler, less expensive components could be employed for the purposes of generating and/or sensing light.